At present, flexography is one of the main printing processes. A flexo sleeve, normally rubber or photopolymer, is fabricated in such a way that the areas corresponding to zones to be inked are geometrically higher than the areas corresponding to zones not to be inked. Contacting the flexo sleeve with an inking roller, such as an anilox roller, inks the flexo sleeve. Only the geometrically higher zones of the flexo sleeve are inked, other areas are not inked. Subsequently, the inked flexo sleeve is brought in contact with a substrate and the inked parts transfer ink on to the substrate, thus producing the desired image on the substrate.
In flexography, there is demand for printing continuous designs such as wallpaper, decoration and gift wrapping paper. In general, such flexography applications use a cylindrical form, usually a printing sleeve or a cylindrical printing cylinder formed by fusing the edges of a sheet together to form a seamless, continuous element. Such continuous printing elements are well suited for mounting on conventional laser exposing engraving equipment such as the Barco Graphics Cyrel® Digital Imager (Barco Graphics, Gent, Belgium) or flexography engravers available from ZED Instruments Ltd. (Hersham, Surrey, England) or Applied Laser Engineering Ltd. (West Molesey, Surrey, England).
When continuous designs are imaged, the continuous designs must be imaged fully seamless, otherwise artifacts become visible in the final print. Artifacts at the seams are especially undesirable because such artifacts repeat with each repeat length (i.e. circumference) of the printing sleeve.
For conventional flexography, a film is wrapped around a sleeve and appropriate methods are applied to transfer the image present on that film to the flexo plate material on the sleeve (e.g., photopolymer). Using such a method, a seam is typically visible where the ends of the film overlap or come close to each other.
FIG. 1A illustrates a flexo sleeve 100 on a drum in a prior-art digital flexography apparatus using an external drum laser output scanner (imager, imagesetter). There are several laser imaging methods known to those skilled in the art to image either rubber sleeves or digital photopolymer sleeves. The flexo sleeve 100 is either mounted on a carrier like a mandrel or a carrier sleeve or cylinder 101. The carrier 101 is mounted directly into a drum laser imagesetter (the whole imagesetter is not shown) where carrier 101 functions as the rotating drum during the imaging process. In a drum laser output scanner, the fast scan direction 120 is in circumference direction (circumference of the sleeve), and the slow scan direction 130 is in axial direction. While the carrier 101 rotates in a fast scan direction shown as the negative direction in FIG. 1A, in the external drum laser imagesetter, one image track 102 is transferred in a fast scan direction shown as the positive fast scan direction. During each revolution of the carrier 101 and flexo sleeve 100 assembly the imaging head 104 slowly moves in a slow scan direction shown as the positive slow scan direction in FIG. 1A. This results in the track 102 following a spiral. In one revolution of the drum, a single first spiral image track 102 is completed. A subsequent spiral image track next to the first image track is transferred to the flexo sleeve 100 in the next revolution. The process repeats until the image is completely transferred along a spiral 102 to the flexo sleeve 100. This process is referred to as a spiral advance imaging process. The case shown in FIG. 1A is of a single laser beam output scanner. A seam is shown as 105 in FIG. 1A.
Multiple laser beam output scanners that follow spiral advance also are known. With a multi-beam system, several tracks are written during each revolution. Thus, the complete image is transferred along several spirals rather than a single spiral.
Modem laser scanning imagesetters usually use spiral advance in the slow scan direction perpendicular to the scan line (“fast-scan”) direction. The spiral shape may not be a problem when imaging plates, not even for multiple beam imaging systems, because correction methods can be applied so that the result is an image that is slightly turned on the printing plate. The plate is usually cut before mounting it on a press sleeve, so the turned image can be compensated for by mounting the finally processed printing plate properly turned in the opposite direction on that press sleeve.
FIGS. 1B–1E show the pixels of one or more spiral advanced scanned lines such as line 102 through several portions, shown in an exaggerated manner as regions 110, 112, 114, and 116, respectively, of the laser scanned flexo sleeve 100 in FIG. 1A on a prior-art spiral advance scanner. The fast scan direction is the same in all FIGS. 1B–1E, and is shown as direction 180 in FIG. 1B.
FIG. 1B shows scanned pixels 140, 142 imaged at location 110 on the flexo sleeve 100 that is not near the seam 105. FIGS. 1C–1E show scanned pixels 150, 152, 154, 156, 160, 162, respectively, imaged at a seam 105.
FIG. 1C shows scanned sets of pixels 152, 154 imaged with a single laser beam on either side of the seam 105. Scanned sets of pixels 152, 154 are offset from each other in the slow scan direction 130 approximately the width of one laser beam.
FIG. 1D shows scanned sets of pixels 154, 156, imaged with two laser beams, on either side of the seam 105. Scanned sets of pixels 154, 156 are offset from each other in the slow scan direction 130 approximately the width of two laser beams.
FIG. 1E shows scanned sets of pixels 160, 162 imaged with four laser beams, on either side of the seam 105. Scanned sets of pixels 160, 162 are offset from each other in the slow scan direction 130 approximately the width of four laser beams.
As can be seen in FIGS. 1B–1E, the spiral advance process results in scanned sets of pixels 150, 152, 154, 156, 160, 162 located near or at the seam 105 being formed differently (i.e. having offsets) from scanned sets of pixels 140, 142 having the same area and located away from the seam 105. The respective offsets shown in FIGS. 1C–1E may result in visible artifacts or errors at the seam 105. The visible artifacts are typically more pronounced if more than one track is imaged at a time, for example using a multiple-beam imaging system. For NB laser beams, this offset can be NB times the distance between two image tracks in the slow scan direction 130, as can be seen in FIGS. 1C–1E.
As shown in FIG. 1C, single laser beam system using spiral advance may not result in severe artifacts. Even in this case, however, the seam 105 may become visible in some screen patterns, especially in homogeneous screens in the middle percentage area (around 50% coverage), for example, screens that use small dots, or for thin, regular vertical lines across the seam (e.g. bar-codes).
When increasing the number of laser beams (FIGS. 1D and 1E), the offset between two adjacent pixels 154, 156 and 160, 162 around the seam 105 becomes larger, and the resulting artifacts become more visible.
Current laser beam drum scanners offer only rudimentary support of seamless imaging of flexo sleeve 100. This is especially true for the emerging multiple beam imaging systems such as the CreoScitex ThermoFlex™ (CreoScitex Division of Creo Products Inc., Vancouver, BC, Canada).
FIG. 2 illustrates one prior-art method, known as “block advance” to reduce the artifacts described above in FIGS. 1C–1E. Examples of prior-art systems using block advance include the Grapholas® System from Barco Graphics/Baasel Scheel Lasergraphics, GmbH, Itzehoe, Germany. Barco Graphics, NV is the assignee of the present invention. In block advance, the advance in the slow scan direction 130 stops periodically during imaging data output. Imaging of each track starts at a specific circumference zero position 220 and stops after one revolution of the flexo sleeve 100 is completed and a complete image track 202 is written. The zero position 220 may or may not coincide with the seam 105 of the flexo sleeve 100. After the imaging stops, the imaging head 104 then moves in the slow scan direction to the next imaging position 104A while the flexo sleeve 100 revolves a complete revolution. Imaging of the next image track 204 begins at the zero position 220.
One of the main disadvantages of the block advance method described in FIG. 2 is that imaging requires approximately twice the time of imaging with spiral advance methods. The increase in imaging time is a result of the imaging being stopped for a full revolution while the imaging head 104 is moved in slow scan direction to the subsequent image track. Imaging is accomplished during one full revolution, without moving the imaging head 104, then imaging is stopped during the next full revolution so that the imaging head 104 can be advanced to the next imaging position 104A.
Digital flexography systems are very expensive. Any reduction in productivity such as reduced imaging throughput and response times, are directly correlated to reduced return on investment.
What is needed is a method to reduce the artifacts of the spiral advance method while still maintaining substantially less loss of productivity than the prior-art block advance imaging method.